Timing and kinematics of deformation in the northwest Adirondack Lowlands, New York State: Implications for terrane relationships in the southern Grenville Province

Abstract

Determining the relationship among crustal blocks within an orogen is a key factor in understanding the architecture and construction of that orogen. Within the large, mid-Proterozoic Grenville Province, the relationship between the Adirondack Lowlands and the adjacent Frontenac terrane is ambiguous. Review of previous work demonstrates that the Adirondack Lowlands have different plutonic suites, a lower grade of metamorphism, and a different geochemical signature. However, the timing and kinematics of deformation in the Lowlands, and their relation to major orogenic events, have not previously been well constrained, making comparisons with the Frontenac terrane diffi cult.On the northwestern edge of the Adirondack Lowlands, detailed structural analysis of upper-amphibolite grade migmatites and marbles reveals two penetrative deformation phases. Interference of F 1 and F 2 folds results in Type 3 fold interference patterns and is suffi cient to produce the regional map patterns. The Noname ductile shear zone, a 0.5-2-km-wide northeast-striking steep ductile shear zone with subvertical lineation, developed during D 2 . The steep geometry of the Noname ductile shear zone, paired with consistent sinistral kinematic indicators only found in subhorizontal surfaces, indicate that kinematics for D 2 was sinistral transpression.Sensitive high-resolution ion microprobereverse geometry (SHRIMP-RG) U-Pb zircon geochronology from three granitic samples that have well-defi ned relationships with D 1 and D 2 indicates that both deformation phases developed through continuous or progressive deformation during ca. 1185-1145 Ma. Zircon geochronology from a quartzite, and the presence of melt during all deformation phases, demonstrate that metamorphism was synchronous with deformation.This work reveals that the Shawinigan orogeny (1190-1140 Ma) developed the dominant structural features observed in the northwest Adirondack Lowlands. These structures are the result of the northward collision of a rifted slice of the Laurentian margin (Adirondis) into previously accreted terranes on the margin of Laurentia. Shawinigan deformation of the Adirondack Lowlands may have outlasted that of the Frontenac terrane across any potential terrane-bounding shear zone. While Frontenac terrane and Adirondack Lowlands geology are suffi ciently distinct to warrant separate terrane designation, evidence is lacking to indicate that a suture exists between them.

Orogenic to postorogenic (1.20-1.15 Ga) magmatism in the Adirondack Lowlands and Frontenac terrane, southern Grenville Province, USA and Canada

“…1.16 Ga. The nature and timing of this deformation is consistent with other studies in this part of the Adirondack Lowlands (e.g., Baird and Shrady, 2011). Wong et al (2011) interpret the BLSZ as accommodating the tectonic juxtaposition of the Frontenac and Adirondack Lowlands terranes during the Shawinigan orogeny, associated with closure of the backarc basin that formed at the edge of Laurentia at 1.28-1.22 Ga (Chiarenzelli et al, 2010b).…”

“…This age was also reported in one multigrain zircon population by McLelland et al (1991), along with older inherited zircon. This 1172 Ma age is contemporaneous with garnet and monazite ages (Mezger et al, 1991), migmatite formation in the metapelitic rocks (Heumann et al, 2006), and penetrative deformation of the Adirondack Lowlands (Baird and Shrady, 2011). These relationships indicate that the current level of exposure was at depth at the time, supporting a plutonic origin for the HSG.…”

“…Geochemical zoning within these bodies was interpreted as relict of volcanic stratigraphy. The domical structure of the HSG bodies has been attributed to syntectonic intrusion, sheath folding, cross-folding, or as being the result of deformation during transpression (see Baird and Shrady, 2011). McLelland et al (1991) suggested that the HSG has a plutonic origin, and that rocks in contact with the HSG show contact-metamorphic temperatures higher than those of regional Shawinigan metamorphic isotherms (see also Hudson, 1994).…”

Tectonic Evolution of the Adirondack Mountains and Grenville Orogen Inliers within the USA

“…1050 Ma but also contains evidence of earlier, ca. 1.6 Ga eastward thrusting (Johnson et al 2004;Selleck et al 2005;Baird et al 2008;Baird and Shrady 2011). The AHT is underlain principally by orthogneiss that is relatively resistant to erosion, while the Lowlands are underlain principally by metasedimentary rocks, notably marble, that are easily eroded thus accounting for the topographic differences between the two terranes.…”

“…7C), and its eastward vergence is recorded by kinematic indicators in the ca. 1.164 Ga Diana Syenite Complex (McLelland et al 2010a, Baird andShrady 2011). Overthrusting of the ALT placed it into the domain of the Adirondack orogenic lid accounting for its preservation of Shawinigan titanite ages (Mezger et al 1991;McLelland et al 1996McLelland et al , 2010a, as well as hornblende 40 Ar/…”